Cardiac defibrillators are implanted in patients suffering from potentially lethal arrhythmias. An implantable defibrillator includes circuity which monitors cardiac activity and decides, based upon the cardiac activity, whether an application is required of defibrillation energy to the heart muscle. If an arrhythmia is detected, pacing or shock therapy may be used to terminate the arrhythmia.
Cardioversion and defibrillation require that a high voltage shock be delivered to the heart muscle. Upon determining a need for such shock treatment, the defibrillator charges a capacitor to a high level, which charge is then delivered as a shock to the heart muscle. The voltage applied across the heart muscle may be as high as 800 volts and the energy applied to the heart muscle is delivered within a very short time period. (e.g. 5 milliseconds).
Early work by Schuder, et al. ("Transthoracic Ventricular Defibrillation with Triangular and Trapezoidal Waveforms", Circulation Research, Vol. XIX, October 1966, pp. 689-694) demonstrated that the pulse shape influences the efficacy of defibrillation. The most commonly held theory on the optimization of defibrillation pulse shape is the average current hypothesis, which is an adaptation of simple cardiac stimulation theory. This general theory states that the efficacy of defibrillation is solely dependent upon the average current of the pulse and the duration of the pulse. This theory implies that other parameters of the pulse are not critical. A recent review of this theory is provided in the article by Mark W. Kroll ("A Minimal Model of the Monophasic Defibrillation Pulse" Pacing and Clinical Electrophysiology, Vol. 16, Part I, April, 1993, pp. 769-777). A more complete review of the influence of pulse shape is provided by W. A. Tacker and Leslie A. Geddes in Electrical Defibrillation (CRC Press, Inc., Boca Raton, Fla., 1980 (Chapter 4)). Based on this theory and the results of these and other published experiments, defibrillation with rectangular pulse has been shown to require less peak current and less energy than is required for any other monophasic pulse shapes. In general, these studies indicate that energy stored at high electrical potential is more effective at defibrillating a heart than the same amount of energy stored at low electrical potential. Further, it has been shown that long, low amplitude tails on the pulse are detrimental to defibrillation.
As capacitors in a defibrillator must today store up to 40 joules, the size they occupy is generally large and they are difficult to package in a small implantable device. Multilayer ceramic capacitors have not been considered for high energy storage applications, even though some exhibit very large dielectric constants. Such high voltage multilayer ceramic capacitors, in large sizes, have been expensive specialty items and their total energy per unit volume is not nearly as high as in electrolytic capacitors. Recently, fabrication technologies for multilayer ceramic capacitors have improved significantly, permitting both substantially higher energy densities and lower costs.
To achieve acceptable performance for a capacitor in a defibrillator, its energy discharge capacitors must be able, for example, to withstand an applied voltage of approximately 800 volts, exhibit a capacitance of approximately 50 to 150 microfarads and occupy a volume of less than 20 cubic centimeters. An energy storage capacity up to 40 joules is required. Ferroelectric ceramics, while having extremely high dielectric constant values at low electric fields, exhibit a significant fall off in capacitance with increasing levels of applied electric field. In fact, at high field values, the exhibited capacitance can be as little as 10% of that at low dc field values. That is, conventional ferroelectric capacitors store a larger portion of their total charge at low voltage as compared with linear dielectric capacitors which maintain a constant capacitance independent of applied voltage. Lastly, ferroelectric materials can also show a substantial hysteresis loss in the charge/discharge, characteristic.
A class of ceramic materials, termed antiferroelectrics, exhibit increasing dielectric constant as electric fields are increased. With further increase in the electric field antiferroelectrics undergo a phase transition to a ferroelectric phase and then show a similar decrease in dielectric constant as is shown by conventional ferroelectrics.
It is an object of this invention to provide an improved, implantable cardiac defibrillator.
It is another object of this invention to provide an improved storage capacitor for use in an implantable cardiac defibrillator.
It is yet another object of this invention to provide an improved capacitor for an implantable cardiac defibrillator wherein the capacitor occupies less volume than prior art capacitors.